1. Field of the Invention
This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a capacitor filter. More specifically, this invention relates to feedthrough terminal pin capacitor filter assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device.
2. Prior Art
Feedthrough terminal pin assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device such as a cardiac pacemaker, a defibrillator, and the like, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure seated in the ferrule. Suitable materials may for the ferrule include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof. The ferrule may be of any geometry including round, rectangle, and oblong.
The terminal pins are typically composed of platinum or a combination of platinum and iridium and provide for passage of electrical signals from the exterior to the interior of the medical device. Platinum and platinum-iridium alloys are biocompatible materials that create a hermetic seal through a gold brazing process that seals any gap between the terminal pin and the supporting insulator. A gold brazing process is also used to hermetically seal any gap between the supporting ferrule and the insulator.
However, too much braze material at the insulator/ferrule interface or at the insulator/terminal pin interface can cause excess tensile stresses leading to cracking of the ceramic insulator with possible subsequent loss of hermeticity. Another problem with hermetic seals at the feedthrough interfaces occurs if the space allotted for the braze material is insufficient to hold or receive its volume. In that case, the braze material can spill out of its interface channel. In addition to causing a weak braze joint and potentially compromising hermeticity, braze spill out presents an esthetic condition that adversely impacts scrap. Also, if an EMI filter is subsequently attached to the feedthrough to attenuate unwanted EMI interference, a flat attachment surface, free of braze spill out, is needed.
Other problems related to excessive braze material at the feedthrough interfaces include unwanted wetting of critical areas. One place this occurs is at the feedthrough perimeter where the ferrule is laser welded to the device shield. Excessive braze material at this interface can compromise the hermetic seal between the ferrule and the device shield.
A second area of concern is on the surface of the insulator between the terminal pins. Over wetting in this area can result in excessive braze material between two terminal pins. This can shorten the pin-to-pin distance which, under high voltage conditions, can cause arcing, or in the case of a filtered feedthrough, premature dielectric breakdown. Another problem is that excessive flow of braze material on the insulator surface can sporadically dewet, then solidify leaving behind small braze balls on the insulator. This unwanted material can also cause arcing or dielectric breakdown under high voltage conditions.
Wetting of the terminal pin below the insulator/terminal pin interface creates another potential problem area. The flex circuit or wire bond substrate leading to the device control circuitry is subsequently welded to the terminal pin here and electrical shorting can occur should the flex circuit come into contact with this excess braze material.
FIGS. 1 and 2 show a feedthrough 10 compromised by excess braze to further illustrate some of these problems. The feedthrough terminal pin assembly 10 comprises a so-called unipolar configuration having a terminal pin 12 extending through a bore in an electrically insulating or dielectric material such as an alumina or fused glass type or ceramic-based insulator 14, hereinafter collectively referred to as an insulator, nested within a ferrule 16. A layer of metal may be applied to the surface of the insulating material, referred to as metallization, to aid in the creation of a brazed hermetic seal. Metallization materials include titanium, niobium, tantalum, gold, molybdenum, silver, platinum, copper, or combinations thereof. The metallization layer may be applied by various processes including sputtering, e-beam deposition, jet vapor deposition, pulsed laser deposition, chemical vapor deposition, plating, electro-less plating or cladding.
The ferrule 16 comprises a cylindrically-shaped body 18 having an upper annular flange 20 extending outwardly along a plane generally perpendicular to the longitudinal axis of the ferrule body. The ferrule body 18 comprises a cylindrically-shaped outer wall sized to snuggly fit in an opening 22 provided in the device shield 24 with the flange 20 resting on an outer surface thereof.
The ferrule body 18 also has a cylindrically-shaped inner wall extending to a lower inwardly-extending annular lip 26 that is sized to receive the insulator 14 in a snug-fitting relationship. That way, the lower end surface 28 of the insulator 14 is coplanar with the lower end surface 30 of the ferrule body. With the insulator 14 seated in the ferrule 16 in this manner, an annulus 32 is formed between them extending along the length of the cylindrically-shaped inner wall of the ferrule body 18 from the annular lip 26 to an annular cut-out channel 34 where the flange 20 meets the body.
A ring-shaped braze pre-form (not shown) is received in the cut-out. Non-liming examples of braze material include gold, particularly gold alloys, and silver. Heating the pre-form causes it to melt and flow into the annulus 32 between the insulator 14 and the ferrule body 18. Upon cooling, the resulting braze 36 now hermetically seals the insulator 14 to the ferrule 16 along the entire length of the annulus 32 and the cut-out 34. However, too much braze material was used and some has spilled out of the cut-out 34, collecting on the upper surface of the ferrule 16. This is shown as braze spill out 36A in FIG. 2.
The insulator further comprises a bore 38 that is sized to provide an annulus 40 between it and the terminal pin 12. A frusto-conically shaped cut-out 42 is provided in the upper surface 44 of the insulator in communication with the annulus 40. A ring-shaped braze pre-form (not shown) is received in this cut-out 42. As with the previously described ferrule braze pre-form, heating the terminal pin pre-form causes it to melt and flow into the annulus 40 between the insulator 14 and the terminal pin 12. Upon cooling, the resulting braze 46 hermetically seals the terminal pin 12 to the insulator 14 along the entire length of the annulus 40. However, too much braze material was used and some has filleted part way up the terminal pin past the upper surface 44 of the insulator 14. This filleting is caused by the difference in the coefficients of friction of the braze material 46 and the terminal pin 12. Braze filleting is undesirable for a number of reasons. Foremost is that it can impair proper attachment of an EMI filter or a molded header assembly to the terminal pin 12 at the upper surface of the insulator. Excess braze material on the terminal pin is also aesthetically unacceptable.
A weld 48 hermetically seals the flange perimeter to the shield 24.
Therefore, there is a need for feedthrough structures that prevent filleting and spill out of braze materials into areas where they can compromise hermeticity as well as prevent the proper attachment of EMI filters and header assemblies to the medical device at the feedthrough. The present feedthroughs provide just such structures.